Embodiments of the invention relate power converters and more specifically to a system and a method for integrating energy storage into modular multi-level power converter.
Energy storage has becoming an increasingly important key element for a variety of power system applications, including motive/transportation and stationary/power grid applications. Traditionally, electrical energy storage such as batteries and ultracapacitors are designed and well suited for direct current (DC) applications that provide active power and load support. Often times, DC to DC power converter will be needed in coupling the energy storage module(s)/element(s) to a DC bus in the power system. For an alternating current (AC) application (e.g., supporting AC loads), the DC bus will then be tied to an AC power system/grid via a DC/AC inverter. This requires multiple stages of power conversion, associated with power conversion efficiency/loss at each stage.
For the DC/AC power conversion, modular multi-level converters (MMCs) are becoming more and more utilized in particular for high and medium voltage applications due to its modular and scalable characteristics. MMCs also have high reliability through the use of redundant modules and high output power quality (e.g., less total harmonic distortion/THD) with little or no filters, thereby reducing system weight and volume or increasing power density.
When MMC is integrated with additional energy storage units, such as batteries and/or super-capacitors/ultracapacitors, it reduces the number of power conversion stages and thus increases overall system efficiency. In addition, when MMC is combined with multiple (or hybrid) energy storage modules in one system, it provides multiple system functions such as uninterrupted power supply (UPS) and transient assist/smoothing functions.
The challenge of using energy storage units (e.g., super-capacitors) within the MMC is the limitation in the amount of current ripple that the energy storage units (e.g., ultracapacitors) can accept without significantly overheating; reducing their useful life; and/or significantly oversizing the energy storage elements. For instance, when the module ultracapacitors are used for a 60 Hz application, they have to conduct significant ripple currents with 60 Hz, 120 Hz, and other frequency content. This oversized energy storage capacity to handle the ripple currents do not support active power/load, and significantly increase/penalize system volume and weight.
For these and other reasons, there is a need for improved system for integrating energy storages with modular multilevel converters.